(a) Field of the Invention
The present invention relates to a process for the production of polyimide fibers having excellent heat resistance, chemical resistance, radiation resistance, mechanical strength and productivity.
(b) Description of the Related Art:
Aromatic polyimide fibers produced by direct heat melt spinning have heretofore been disclosed only once before in Japanese Patent Application Laid-Open No. 211319/1988, by the present assignee. This publication discloses high-strength and high-elasticity polyimide fibers made of a novel polyimide which permits melt spinning and has good processability. However, the above polyimide fibers are accompanied by problems such that the discharge rate tends to decrease upon long-time continuous spinning due to thickening of the molten resin and/or clogging of the filter and end breakage also takes place frequently upon spinning or stretching. They are therefore still not satisfactory in productivity.
In the meantime, the present assignee filed an application with the Japanese Patent Office (Japanese Patent Application No. 266191/1987). The application claims a specific process for the production of a polyimide, wherein the production process comprises conducting the reaction between a specific diamine and a particular tetracarboxylic dianhydride in the presence of a specific dicarboxylic anhydride. This process can provide a polyimide in which each high-molecular chain is blocked at both ends thereof with the dicarboxylic anhydride. A resin which can be obtained by thoroughly pre-drying the polyimide to adjust its water content to less than 200 ppm, preferably to less than 50 ppm does not undergo any substantial flowability reduction even when exposed to high temperatures for a long time, so that the resin has excellent processability in melt forming such as injection molding and extrusion.